MEMS devices, including bolometers and gyroscopes, require that the devices are sealed at very low pressures of from 1 to 100 μBar to achieve increased sensitivity and to thereby provide a good signal to noise ratio. Typically these devices are micro fabricated on the MEMS wafer and are sealed inside a cavity that is formed as a recess in a cap wafer. The MEMS and cap wafers are typically bonded together using various wafer bonding schemes like eutectic bonding or thermo compression or glass frit bonding. In all of these bonding scenarios, the sealed cavity is approximately at the same pressure level as is present during the bonding of the wafer. The typical pressure levels to achieve bonding is carried out is approximately 1 mBar. As a result, there is a need to further reduce the pressure in the sealed cavity up to three orders of magnitude down to 1 μBar. Typically, “Getters” are used for this purpose. The inner walls of the sealed cavity are coated at selective locations with a getter material. A getter material is one that combines with the gas molecules in the cavity either chemically or via adsorption and increases the level of vacuum (or decreases the pressure) in the cavity. At times these getter materials might require an elevated temperature to “activate” gettering of the gas molecules.
The efficiency of a getter is strongly dependent on the surface area which is exposed to the gas molecules. The larger the surface area, the more gas molecules are adsorbed into the getter and thereby evacuated from the chamber leading to lowered pressure levels. Since the available surface area on the inside of a sealed cavity is limited, the surface area for creating a getter having improved performance is limited. Deep trenches on the interior surface of the sealed cavity using processes such as Deep Reactive Ion Etching (DRIE) could be implemented to increase the surface area to some extent. However, the need for more efficient getters continues to exist as the MEMS packages continue to shrink in size (especially in Consumer Electronics). Of course, smaller MEMS packages have reduced surface areas and therefore there is less area in which the trench patterns can be formed by DRIE.
Consequently, there is a need for a getter which can reduce the pressure in the sealed cavity to desirable levels. By reducing the pressure in the sealed cavity, better performance of the MEMS devices can be achieved.